Picosecond fluorescence of intact and dissolved PSI-LHCI crystals.
Over the past several years, many crystal structures of photosynthetic pigment-protein complexes have been determined, and these have been used extensively to model spectroscopic results obtained on the same proteins in solution. However, the crystal structure is not necessarily identical to the structure of the protein in solution. Here, we studied picosecond fluorescence of photosystem I light-harvesting complex I (PSI-LHCI), a multisubunit pigment-protein complex that catalyzes the first steps of photosynthesis. The ultrafast fluorescence of PSI-LHCI crystals is identical to that of dissolved crystals, but differs considerably from most kinetics presented in the literature. In contrast to most studies, the data presented here can be modeled quantitatively with only two compartments: PSI core and LHCI. This yields the rate of charge separation from an equilibrated core (22.5 +/- 2.5 ps) and rates of excitation energy transfer from LHCI to core (k(LC)) and vice versa (k(CL)). The ratio between these rates, R = k(CL)/k(LC), appears to be wavelength-dependent and scales with the ratio of the absorption spectra of LHCI and core, indicating the validity of a detailed balance relation between both compartments. k(LC) depends slightly but nonsystematically on detection wavelength, averaging (9.4 +/- 4.9 ps)(-1). R ranges from 0.5 (<690 nm) to approximately 1.3 above 720 nm.
van Oort
Bart
B
Laboratory of Biophysics, Wageningen University, 6703 HA Wageningen, The Netherlands.
Amunts
Alexey
A
Borst
Jan Willem
JW
van Hoek
Arie
A
Nelson
Nathan
N
van Amerongen
Herbert
H
Croce
Roberta
R
D018360:Crystallography, X-Ray; D005453:Fluorescence; D007700:Kinetics; D045342:Light-Harvesting Protein Complexes / Q000737:chemistry* / Q000378:metabolism*; D008856:Microscopy, Fluorescence; D018532:Pisum sativum / Q000201:enzymology*; D045331:Photosystem I Protein Complex / Q000737:chemistry* / Q000378:metabolism*; D012996:Solutions; D013997:Time Factors
D045342:Light-Harvesting Protein Complexes; D045331:Photosystem I Protein Complex; D012996:Solutions